I recent created an .OLM file to save my 2012 emails which were all in one folder; unfortunately the .OLM file created also contained 2011 emails which were open on another .OLM file I had opened, plus first few days of 2013.

Not in itself a disaster, but a larger file than I wanted at 15 GB

After some shenanigans I got a clean Identity where I managed to pull in only 2012 emails from that 15 GB OLM file, and then resaved a new .OLM file.

Should be smaller - right ? No ! It has now grown to 17 GB ! I do not see how as it now does not have all the emails the original had.